When I started with polymerized tung oil (PTO) I used the Lee Valley brand. Really like that as it would dry overnight so was relatively quick to build coats. I'd also mention that the cotton scraps I used for application would also turn "stiff/hard" overnight.
Started investigating and found that Sutherland Welles made the tung oil for Lee Valley. Some people claimed that the Sutherland Welles oil didn't use chemical driers, but found it hard to believe that the only thing they did was heat polymerize the oil and it would still harden overnight. Dug and dug and eventually found some MSDS/SDS documents that listed proprietary chemical driers in their formulation.
Chemical driers brings up the whole question of "food safe" which has as many opinions as there are people discussing it. I did a deep dive into the regulations that are posted here:
https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-175
Needless to say, you almost need to be a lawyer and a chemist to understand everything there. Bottom line is chemical driers, cobalt included, are listed in the "generally accepted as safe" category when discussing finishes with food contact. The main caveat is the finish must be fully cured, with PTO that is roughly a month, depending on conditions.
In layman's terms, chemical driers work by attracting more O² and thus catalyzing the oxidative polymerization (drying and curing) of the finish.
I won't argue for or against the "food safe" nature of a finish with chemical driers, rather putting all the info out there for each to decide.
Lots of preamble to your simple question. Several years ago, to save money I started playing to formulate my own PTO (using driers) that would act the same as the Lee Valley I had been using. I'll find the thread and repost it here with all the details.
Short story is I got some heat polymerized tung oil and an equal amount of citrus solvent from The Skin Boat Store
https://www.skinboats.com/?srsltid=AfmBOorKLb0Sx0pSgjW8RKgpu5n3APv7g1UuWfrqr5mXbCqeufwOILOC
The consistency of the PTO was that of dark corn syrup, so you definitely need a solvent to cut it.
Tried using their PTO thinned 50:50 as a baseline and it took over a week to dry. That proved to me that a tung oil that was heat treated alone would not perform like the Lee Valley oil I had been using. At that point, I started playing with mixing small batches with Japan Drier and testing dry times. This thread detailing my experiments is here:
I mentioned in the other thread on polymerized tung oil (PTO) that I use it on a majority of my turning projects except pens and a few other spindle projects. To date, I've only used PTO from Lee Valley, which is rather expensive at $51 per liter. Although expensive to buy, very little is used...
woodbarter.com
I'm still using some of the original batch I made in October 2020 (stored in stop loss bags) and it's performing now as it did then.
A couple added notes for clarification. There are other chemical driers that are being used that contain no cobalt. However, they are harder (more expensive?) to find and mention has been made that they can have a lot of yellowing develop.
I don't know what Sutherland Welles uses for their driers. Have no idea if they use cobalt or not.
If anyone decides to make their own PTO with Japan Drier be aware of the toxicity and use appropriate PPE.
Long post, but it can get a little complicated with all the potential issues.
Final comment, I'm not a chemist, nor a toxicologist, or a lawyer so make your own decisions. This is just what I've done.
